This invention relates to a method for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation containing low-gravity viscous oils or bitumens. More particularly, this invention relates to recovery of hydrocarbons from tar sands.
The recovery of viscous oils from formations and bitumens from tar sands by conventional methods has generally been unsuccessful because of the high viscosity and low mobility of the oil or bitumens. While some success has been realized in stimulating recovery of heavy oils by the use of thermal methods, essentially no success has been realized in recovering bitumens from tar sands. Bitumens can be regarded as highly viscous oils having a gravity in the range of about 5.degree. to 10.degree. API and contained in an essentially unconsolidated sand. These formations containing bitumens are referred to as tar sands. One such deposit is the Athabasca tar sands located in Alberta, Canada, which is estimated to contain some seven hundred billion barrels of oil.
Among the conventional thermal recovery methods applied to produce viscous hydrocarbons from formations and bitumens from the tar sands are steam injection, hot water injection and in-situ combustion. Using these thermal methods, the in-situ hydrocarbons are heated to temperatures at which their viscosity is sufficiently reduced and their mobility is sufficiently improved so as to enhance their flow through the pores of the formation.
Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation. In a steam operation the heat furnished by the injected steam functions to lower the viscosity of the oil, thereby improving its mobility, while the fluid flow of the steam through the formation functions to drive the oil toward the production well from which the oil is produced.
In the conventional in-situ combustion operation, characteristically much higher temperatures, i.e. above the ignition temperature of the crude, are obtained than in a steam operation. An oxygen-containing gas such as air is injected into the formation and combustion of a portion of the in-place crude adjacent the wellbore is initiated by one of many accepted means, such as the use of a downhole gas-fired heater or a downhole electric heater or chemical means. After initiation of combustion has occurred, the injection of the oxygen-containing gas is continued so as to maintain a combustion front which is formed and to drive the front through the formation toward the production well. As the combustion front moves through the formation, the hot gases and liquids moving in advance of the combustion front vaporize the volatile components of the formation fluids and displace them ahead of the front. Only the higher boiling components of the oil remain and they serve to provide fuel for continuation of the process. The volatilized components move in the vapor phase until they reach a zone where the composition and temperature of the formation are such that they are either condensed or absorbed in the oil.
Another technique that has been employed to recover viscous hydrocarbons is the use of hydrocarbon solvents. For example, it is well known that aromatic solvents, such as toluene and benzene, are capable of dissolving the heavier hydrocarbon components in heavy oils or bitumens, thereby improving their mobility by dilution. Aromatic solvents are generally more effective than paraffinic-type solvents since the asphaltic components of the oils are less soluble in paraffinic solvents. The solvents have a beneficial result in that they dilute the crude and thus make the crude more mobile. However, their use has not been practical commercially since their cost is high and recovery of the solvent tends to be low.
It is also known to inject hot solvent into the formation to accomplish a hot solvent extraction. However, surface fuel and expensive surface equipment are required. In addition, surface heating is relatively inefficient and rather elaborate and rigorous procedures are required because of the possibility of fires and explosions.
Among the difficulties that arise in the practice of thermal methods of recovery is the lack of conformance. Conformance is defined as the volumetric fraction or percent of the oil-bearing formation that is invaded or swept by the injected fluid or swept by the injected fluid or fluids in secondary recovery operations. Conformance is also expressed in terms of horizontal and vertical sweep efficiencies. It is the most inefficient parameter of a recovery operation. The injected fluid follows the path or paths having the highest transmissibility, which could represent a very small fraction of the total reservoir. For example, in the in-situ combustion process, the fronts are propagated at velocities that cause them to pass preferentially through the more permeable areas of the formation and bypass the less permeable areas. Thus, there are some unburned areas from which no oil is recovered. There is also the undesirable result that, with the passage of each successive front, the tendency of the oxygen-containing gas to follow previously created channels increases. Thus, the efficiency of the process is low and it continues to decrease if the injection and production are continued.
One suggestion for improving conformance is the injection of water either simultaneously or intermittently with the oxygen-containing gas, whereby conformance is improved by readjusting the mobilities of the fluids to a more favorable ratio. However, this method has not been too successful, particularly in reservoirs having numerous permeability streaks or in formations containing viscous oils. This is particularly true with tar sands.
It is thus an object of my invention to provide a recovery process wherein improved conformance is obtained by exploiting the advantages of creating thermal and compositional gradients in the formation. This improved conformance, which results in enhanced recovery, is obtained by the injection of a hydrocarbon solvent, that is saturated with a gas and thereafter establishing a heat wave in the formation. The formation then is subjected to a soak period after which it is produced to recover the hydrocarbons therein.